GAS TURBINE POWER APPLICATION

The first gas turbine in production for electrical power generation was introduced by Brown Boveri of Switzerland in 1937. It was a standby unit with a thermal efficiency of 17%. Today the gas turbine is a major player in the huge power generation market, with orders of around 30GW per year. This success is due partly to large reserves of natural gas which provide a cheap fuel which is rich in hydrogen, and therefore produces less carbon dioxide than liquid fuels. The other major factor is thermal efficiency, which for combined cycle power plants approaches 60%. A final advantage is the viability of gas turbines in a very wide range of power levels, up to 300MW per engine for simple cycle and 500MW in combined cycle. The market is split evenly between 50 Hz areas such as much of western Europe and the former Soviet Union, and 60 Hz sectors such as North AmericaSome classes of power generation application:-Plant type Examples of

applicationsExamples of engine

Power perengine (MW)

Peak lopping Supply to grid Alstom GT10 20–60

units,simple cycle gasturbine

RR RB211GE LM600

Mid merit powerstation, simple cyclegas turbine

Supply to grid GE LM6000RR Trent

30–60

Base load powerstation, gas turbine incombined cycle

Supply to grid WEC 501FGEPG9331(FA)

50–450

Base load powerstation, coal firedsteam plant

Supply to grid 200–800

Base load powerstation, nuclearpowered steam plant

Supply to grid 800–2000

RR=Rolls-Royce WEC=Westinghouse Electric Company (now part of Siemens)GE=General ElectricCOMBINED CYCLE POWER PLANTS:

a. Definition. In general usage the term ‘ ‘combined cycle power plant” describes the combination of gas turbine generator(s) (Brayton cycle) with turbine exhaust waste heat boiler(s) and steam turbine generator(s) (Rankine cycle) for the production Of electric power. If the steam from the waste heat boiler is used for process or space heating, the term "cogeneration” is the more correct terminology (simultaneous production of electric and heat energy).b. General description. (1) Simple cycle gas turbine generators, when operated as independent electric power producers,are relatively inefficient with net heat rates at full load of over 15,000 Btu per kilowatt-hour. Consequently, simple cycle gas turbine generators will be used only for peaking or standby service when fuel economy is of small importance.(2) Condensing steam turbine generators have full load heat rates of over 13,000 Btu per kilowatthour and are relatively expensive to install and operate. The efficiency of such units is poor compared to the 8500 to 9000 Btu per kilowatt-hour heat rates typical of a large, fossil fuel fired utility generating station.(3) The gas turbine exhausts relatively large quantities of gases at temperatures over 900 “F, In combined cycle operation, then, the exhaust gases from each gas turbine will be ducted to a waste heat boiler. The heat in these gases, ordinarily exhausted to the atmosphere, generates high pressure superheated steam. This steam will be piped to a steam turbine generator. The resulting “combined cycle”heat rate is in the 8500 to 10,500 Btu per net kilowatt- hour range, or roughly one-third less than a simple cycle gas turbine generator.

(4) The disadvantage of the combined cycle is that natural gas and light distillate fuels required for low maintenance operation of a gas turbine areexpensive. Heavier distillates and residual oils are also expensive as compared to coal.

combined cycle plantUnit 4-2 at Higashi Niigata Thermal Power Station ofTohoku Electric Power Co., Inc.

Small scale combined heat and power – CHP:-

In this application the waste heat is typically utilized in an industrial process. The heat may be used directly in drying processes or more usually it is converted by an HRSG (heat recovery steam generator) into steam for other uses. Most CHP systems burn natural gas fuel. The electricity generated is often used locally, and any excess exported to the grid. The key power plant selection criteria in order of importance are:(1) Thermal efficiency, for both CHP and simple cycle operation. The latter becomes more significant if for parts of the year there is no use for the full exhaust heat.(2) Heat to power ratio is important as electricity is a more valuable commodity than

heat. Hence a low ratio is an advantage as the unit may be sized for the heat requirement and any excess electricity sold to the grid.(3) The grade (temperature) of the heat is very important in that the process usually demands a high temperature. (4) Owing to the high utilisation, low unit cost, start and acceleration times are all of secondary importance, as are weight, volume and part speed torque. The attributes of the gas turbine engine best meet the above criteria, and hence it is the market leader. The diesel engine still retains a strong presence however, particularly for applications where substantial low grade heat is acceptable, or where the importance of simple cycle thermal efficiency is paramountThe microturbine market has emerged in recent years with a number of forecasts predicting dramatic growth. Small gas turbines of between 40kW and 250kW are installed in buildings, such as a store or restaurant, to generate electricity and provide space heating and hot water. A connection with the grid for import/export is usually maintained. The very small size of microturbine turbomachinery leads to low component efficiencies and pressure ratio,

hence to achieve circa 30% thermal efficiency the gas turbine must be recuperated. Otherwise the configuration is extremely simple as low unit cost is critical. Usually it comprises a single centrifugal compressor, DLE ‘pipe’ combustor, either a radial or two stage axial turbine and the recuperator. Another key feature is a directly driven high speed generator – the size of a gearbox to step down from the turbomachinery speed of typically 90,000 rpm to 3000/3600 rpm is impractical. This also requires power electronics to rectify the ‘wild’ high frequency generator output into DC, and then convert it back to 50 Hz or 60 Hz AC.Large scale CHP:-Here the waste heat is almost exclusively used to raise steam, which is then used in a large process application such as a paper mill, or for district heating. Again the electricity generated may be used locally or exported to the grid. The importance of performance criteria to engine selection are as for small scale CHP, except that emissions legislation is more severe at the larger engine size. Here gas turbines are used almost exclusively. High grade heat is essential, and the weight and volume of diesel engines prohibitive at these power outputs. Furthermore the gas turbines used

are often applicable to other markets, such as oil and gas, and marine, which reduces unit cost. Aero-derivative gas turbines are the most common, though some heavyweight engines are used. Aero-derivatives usually employ the core from a large civil turbofan as a gas generator, with a custom designed free power turbine for industrial use. Heavyweight engines are designed specifically for industrial applications and as implied are far heavier than aeroderivatives, their low cost construction employing solid rotors, thick casings, etc. The gas turbine configuration is usually a free power turbine. While this is not necessary forCHP applications, it is essential to also allow use in oil and gas and marine. Axial flow compressors are used exclusively with overall pressure ratios between 15 :1 and 25 :1. The aero-derivatives are at the top end of this range as this pressure ratio level results from a civil turbofan core. This pressure ratio is a compromise between that required for optimum CHPthermal efficiency of 20 :1, and the 35 :1 for optimum simple cycle efficiency. These values apply to the typical SOT of between 1450K and 1550 K. Advanced cooling systems are employed for at least both the HP turbine first stage nozzle guide vanes and blades.

Applications which supply solely to a grid system:-Power plants supplying a grid fall into three categories:(1) Peak lopping engines have a low utilisation, typically less than 10%. They are employed to satisfy the peak demand for electrical power which may occur on mid-weekday evenings as people return home and switch on a multitude of appliances.

(2) Base load power plant achieve as near to 100% utilisation as possible to supply the continuous need for electrical power.(3) Mid merit power plant typically have a 30–50% utilisation. They serve the extra demand for electricity which is seasonal, such as the winter period in temperate climates where demand increases for domestic heating and lighting. The considerations in selecting the type of power plant for a base load power station are as follows.(1) Thermal efficiency and availability are paramount.(2) Unit cost is of high importance as the capital investment, and period of time before the power station comes on line to generate a return on the investment are large.(3) Cost of electricity is a key factor in selecting the type of power plant, and fuel

price is a major contributor to this. Coal, nuclear and oil fired steam plants all compete with the gas turbine. In all cases weight and volume are of secondary importance. Other specific comments are as follows: (1)For base load plant, start and acceleration times are unimportant. .(2) For peak lopping power stations unit cost is crucial, time onto full load is very important and thermal efficiency relatively unimportant. (3)Mid merit power stations are a compromise with some unit cost increase over and above peak loppers being acceptable in return for a moderate gain in thermal efficiency.

Application in nuclear field:-Nuclear-assisted natural gasturbine combined cycle (NGC C ):-Using a combination of a nuclear reactor, which emits no carbon dioxide, and a high efficiency gas turbine cycle, electric utilities can reduce generation cost as well as minimize the greenhouse gas emissions.Nuclear power is an important source of almost CO2-free electricity that does not rely on fossil fuel and hence does not produce greenhouse gas emissions. However, nuclear power plants have low energy efficiency

compared to that of modern fossil power plants. The conventional pressurized water reactor using subcritical Rankine steam cycle has a thermal efficiency of 30-35%, but the conventional gas turbine based combined cycle has thermal efficiency of 55-60%. In addition to these pros and cons of nuclear and fossil power, there are also other aspects as shown in the fowlloingTable.

Nuclear power (PWR/GCR)

Fossil power (natural gas)

ProsLow/NoCO2

emission

ConsHigh CO2 emissions

High fuel costLow fuel cost

ConsLowthermal efficiency

ProsHighthermal

efficiency

High capital cost Low capital cost

The main motive for development of the nuclear assisted combined-cycle is adding the advantages of nuclear power to the

advantages of fossil power. By synthesizing these pros of nuclear and fossil powers, we can achieve highly efficient power plant with low greenhouse gas emissions.

Table –A. Pros and cons of nuclear and fossil powers

Figure A. Schematic of the nuclear assisted combined cycle

As shown in Figure A, an advanced gas cooled nuclear reactor is added to the conventional NGCC. The upper left part of the figure shows the nuclear reactor along with a heat exchanger and a circulator, and the right part depicts a conventional NGCC. In this configuration, the nuclear reactor plays the role of a pre-heater for the gas combustor.

The compressed air from the compressor outlet is heated up to T5 by the hot gas exiting from the nuclear reactor at TRO. The pre-heated air is entered into the combustor where natural gas combustion occurs. So, there is no need for a balance of plant for the reactor. Referring to Figure B, the savings in fossil fuel because of the nuclear reactor is

(1)

With fixed T3 and T2, a higher value of T5 would yield higher savings in fossil fuel.

Figure B. T-s diagram of the nuclear assisted combined cycle

Contribution of nuclear power to the nuclear-assisted NGCCThe nuclear contribution to the system depends on the nuclear-heated gas temperature. As shown in Table B and Figure c, the contribution of nuclear power increases as the reactor outlet gas temperature increases. Considering the up-to-date design of PBMR (see Table c) [Pebble Bed Modular Reactor Ltd., 2004], we assume the temperature of nuclear-heated gas as 900ºC for comparing the economic competitiveness with pure NGCC and pure nuclear options.

By assuming 900ºC, the contribution of the nuclear power to the nuclear assisted NGCC is 46.3%, which is equivalent to 178.5 MWe and 298.0 MWth. The total power of the nuclear assisted combined-cycle plant is 382 MWe. For economic comparison, we also assumed the same power for pure nuclear and pure NGCC options.Table-B. The contribution of nuclear power to total energy input

Figure c. Contribution of nuclear power to total energy input

Table c. Main design parameters of conventional PBMR

Cycle efficiency of the nuclear-assisted NGCC:-With a perfect heat exchanger and no work for the helium circulator, the thermal efficiency of the nuclear assisted NGCC would be the same as that of combined-cycle without nuclear power.But, there is a loss in the cycle efficiency due to power required for the circulator in the nuclear reactor loop. This loss depends on the pressure drop through the core and heat exchanger. From the literature the pressure loss through the core is 0.05 MPa [LaBar 2002], and the loss through the heat exchanger is 0.16 MPa [Kadak et al., 2001]. Therefore, the total pressure drop is 0.21 MPa. The mass flow rate in the core is 116.7 kg/s and the volume flow rate is 20.9 m3/s.

Assuming the Helium coolant as an incompressible fluid because of small pressure drop in comparison to the system

pressure of 8.0 MPa, we can calculate the work for this circulator as:

(2)

If we assume 85% efficiency for the circulator, the required power is 5.17 MWe. Then, the cycle efficiency of the nuclear assisted NGCC cycle is 59.1%. Based on this efficiency, the economic analysis has been performed.

Reference:-Book: Gas Turbine Performance - Second Edition

for:Philip P. WalshBSc, FRAeS, CEngHead of Performance and Engine SystemsRolls-Royce plcPaul FletcherMA (Oxon), MRAeS, CEngManager, Prelim DesignEnergy BusinessRolls-Royce plc

Publisher:-

Web site: 140.194.76.129/publications/armytm/tm5-811-6/c-8.pdf

Paper:-For: Y.H. Jeong, P. Saha and M.S. Kazimi

MIT-NES-TR-003

NUCLEAR ENERGY AND SUSTAINABILITY (NES) PROGRAM